Measuring is "judging against a standard." When we measure a thing, we take something with a fixed proportion - like a length or a weight that is known or standardized - and we compare it to whatever we are trying to quantify or measure. We see how it measures up: how long it is, how wide it is, how tall it is, how heavy it is, etc. We can also see if it fits a pattern or a template that is necessary for the item to do its part. In our case, one can see if he is fit for the Kingdom of God.
"For the time has come for judgment to begin at the house of God" (I Peter 4:17). We are being measured, judged, against a standard. "The house of God" is another way of saying "the temple of God," the phrase used here. Judgment begins at the house of God, and the Two Witnesses are given the responsibility of measuring the Temple of God. The two verses are saying basically the same thing. Note, the Two Witnesses are not actually doing the judging - Christ is, for that is His job. The Two Witnesses' responsibility is to explain the basis for the measurements. In other words, it is their job to show what the standard is, to let people know what they should be measuring up to.
Their job is similar to Amos' vision of the plumb line (Amos 7:7). The plumb line can be said to be slightly different because it is used to measure verticality - to see whether something is standing up straight, or to use a more "religious" term, to see if it is "upright." A plumb line is a weight suspended on a string. When it stops swaying like a pendulum, the string is perfectly vertical. When a workman puts it next to something like a wall or post that needs to be vertical, he can tell whether his wall or post is out of plumb or not.
That idea is present here in Revelation 11:1. How close do we meet the standard? How upright are we? How fit are we for the Kingdom of God? Finding the answers to these questions is part of the Two Witnesses' job. Remember that the work of the church is essentially done by this time. This preaching of the standard is a work that the ministry of the church has been given to do in every time, but maybe not to this extent. In any event, the Two Witnesses, at this time of the end, are the only ones able to do this job in a major way.
It is possible that this part of their ministry begins, however, before the Seven Thunders cease. In fact, it is a pretty good bet that they will already be involved in ministry before the Tribulation begins. Then God will say, "Okay, now it's time for you to do your real job." They will then begin their prophesied ministry, which will be quite intense.